CTCSS (which stands for "Continuous Tone-Coded Squelch System") has long been used to provide some degree of selective calling and spurious signal rejection in two-way RF communications systems.
Two-way RF communications systems may typically include non-trunked RF repeaters that are "open" in the sense that any RF signal presented to the repeater input RF frequency will be received (and potentially retransmitted and/or regenerated) by the repeater on the repeater RF output frequency. Unless precautions are taken in such systems, it is possible for all transceiver users in the field to be disturbed by spurious transmissions (or even noise from adjacent channels or the like) on the repeater input frequency. While this situation may be acceptable for certain classes of users (e.g., amateur radio operators and other casual users), it is unacceptable for business and public service users who require a higher degree of communications privacy and reliability.
As is well known, CTCSS provides additional, subaudible analog tone signalling over voice channels in order to implement selective calling and/or selective transceiver unsquelching. Briefly, CTCSS involves dividing the audio spectrum of a standard RF channel into two portions: a subaudible "channel guard" portion (e.g., having a frequency range of about 30 Hz to slightly less than 300 Hz) speech portion (typically having a frequency range of 300 Hz to about 3 KHz or more). Speech signals received by a user transceiver via the user's microphone are band limited (typically using a 300 Hz to 3000 Hz bandpass filter) such that no significant components of the speech signal exist below 300 Hz. This lower frequency "channel guard" portion of the audio channel spectrum is reserved for CTCSS tone frequencies generated by a tone encoder included within each authorized transceiver operating on the system. When an authorized user transmits, his transceiver automatically modulates the channel guard portion of the RF envelope with one or more CTCSS subaudible tones in addition to modulating the RF envelope with the user's speech signals.
When such an authorized transmission is received by a base or repeater station, the base station demodulates the received RF signal and uses filtering techniques to separate the demodulated audio spectrum into the channel guard portion and the speech portion. The base station then detects ("decodes") CTCSS signalling with the channel guard portion of the received audio to determine whether one or more CTCSS tones are present. If the base station finds no (or only inappropriate) tones, it may simply ignore the received RF signal (thus helping to prevent unauthorized users from accessing the communications system and also preventing base station falsing on noise from adjacent channels or the like).
If the base station successfully decodes an expected CTCSS tone within the channel guard portion of a received RF signal, the base station typically regenerates a further CTCSS tone (which may be of the same or different frequency as the CTCSS tone received by the base station) and transmits this regenerated tone in the channel guard portion of a transmitted RF output signal (the received detected speech signals are simultaneously retransmitted over the speech band portion of this RF output signal).
The base station RF output signal encoded with the CTCSS signal is received by user transceivers in the field. Each user transceiver also includes a CTCSS tone decoder which controls the transceiver squelch function. While the transceiver (if activated) will typically receive all of the base stations transmissions (as well as any RF noise or other RF transmissions which may happen to be on the received frequency), only those signals encoded with appropriate CTCSS tones will cause the transceiver to unsquelch. Thus, users in the field will only hear authorized base station transmissions.
CTCSS provides a variety of different standard frequency tones (e.g., 67 Hz, 210.7 Hz, etc.), and different transceivers may be unsquelched by different tones (thus providing a limited selective calling feature in some systems). For example, the transceivers of Group A users might encode and decode a CTCSS tone of frequency X, while the transceiver of Group B users might encode and decode a CTCSS tone of frequency Y. If the base station were capable of alternately decoding and encoding tone X and tone Y, Group A base station retransmissions would only be received by the users in Group A, and Group B base station retransmissions would only be received by the users in Group B. Group privacy can thus be provided within a non-trunked repeater communications system.
Such RF communications systems as described above are typically operable in the "full duplex" mode. That is, base station can receive and transmit at the same time, and thus uses different input and output RF frequencies (and includes conventional RF filters and/or other combinet arrangements) so as to permit simultaneous RF transmission and reception. This full duplex capability poses an additional constraint not present in simplex or half-duplex RF communications systems, however, and generally prevents significant sharing of components between transmit-related and receive-related signal processing circuitry.
For example, full duplex RF transceivers must include separate audio amplifier stages for processing received audio signals and transmit audio signals, separate local oscillator components for producing transmit and receive local oscillator frequencies, etc. (as opposed to half-duplex type transceivers which may share and multiplex such components between receive and transmit modes). Of course, even in such full duplex RF communications systems, typical user transceivers are usually only "half duplex" (i.e., transmitting and receiving on different RF frequencies, but incapable of transmitting and receiving simultaneously) in order to decrease cost by permitting circuit sharing between transmit and receive functions.
As one example of such half-duplex arrangements, prior art GE DELTA and RANGR II lines of mobile transceivers include microprocessor-controlled, digitally programmable CTCSS tone encode and decode features. See, for example, the following prior GE publications:
Maintenance Manual 136-174 MHz Synthesizer/ Interconnect Board 19D900961G13,15 Wideband, LBI317733A, 1990; PA1 GE Mobile Comm., MASTR II, Station Programmable Channel Guard (Encode Only) 19C331044G1, Maintenance Manual, 1982; PA1 GE Mobile Comm., MASTR II, Programmable Channel Guard 19D432500G1-3, Maintenance Manual, 1981; and PA1 Mobile Radio, Instructions for Multi-Tone Channel Guard Encoder 19D423094G1 Options 9564-9570, General Electric, 1976. PA1 U.S. Pat. No. 4,376,310 Stackhouse Mar. 8, 1983; PA1 U.S. Pat. No. 4,484,355 Henke et al Nov. 20, 1984; PA1 U.S. Pat. No. 4,171,516 Challen et al Oct. 16, 1979; PA1 U.S. Pat. No. 4,654,881 Dolikian et al Mar. 31, 1987; PA1 U.S. Pat. No. 4,627,098 Dolikian et al Dec. 02, 1986; PA1 U.S. Pat. No. 4,455,617 Dolikian Jun. 19, 1984; PA1 U.S. Pat. No. 4,554,542 Dolikian Nov. 19, 1985; and PA1 U.S. Pat. No. 4,463,221 Soulliard et al Jul. 31, 1984.
However, since these transceivers are only "half duplex" (i.e., they alternately transmit and receive), they do not need to simultaneously encode and decode CTCSS signalling. During transmit, the microprocessor within such prior art transceivers generates CTCSS signalling under software control; and during receive, the microprocessor decodes CTCSS signalling under software control (in each case, the specific CTCSS signalling frequency may be specified by data stored in a personality PROM within the transceiver).
As will be explained, however, such prior art arrangements cannot be readily adapted for full duplex systems. Thus, in the past it has been necessary to provide separate hardware circuits in full duplex type systems where simultaneous encoding and decoding of CTCSS tones is required.
In fact, it was not so long ago that it was necessary to provide a separate hardware component or module for each different CTCSS frequency tone to be decoded by a full duplex base station; and a further separate RF component or module for each different CTCSS frequency tone to be encoded by the base station. Such hardware-based CTCSS tone encoding and tone decoding components were generally not programmable or otherwise easily alterable for different CTCSS tone frequencies and (although perhaps in some cases similar to one another structurally) had to be customized via some physical process (e.g., manual tuning of an oscillator, installation of an appropriate mechanical filter, etc.) in order to operate on a particular desired CTCSS frequency.
Digital technology (e.g., digital signal processors, microprocessors, digitally programmable tone synthesizers, and digital filtering techniques) has made such frequency-specific hardware components largely obsolete. State-of-the-art CTCSS tone encoding and decoding is now typically performed with software-programmable, digitally-responsive encoders and decoders. The following is a non-exhaustive (but somewhat representative) listing of prior-issued U.S. patents relating to digital tone encoding/decoding (some specific to CTCSS):
The Henke et al patent relates to CTCSS tone detectors and generators controlled by a digital data controller. In this Henke et al arrangement, a tone frequency synthesizer and a digitally programmable filtering network external to a data controller are used to programmably encode and decode CTCSS tones in response to digital signals provided by the data controller. The data controller, in turn, selects the particular CTCSS tone frequencies for encoding and decoding in response to information stored in a PROM (programmable read only memory).
The Dollklan et al arrangements disclosed in the '098 and '881 patents cited above relate to channel guard tone signalling over a wire line. These patents disclose a Motorola MC6803-1 microprocessor type digital controller connected with an external digital tone decoder and an external digital code encoder (see FIG. 4). The microprocessor writes to the digital tone decoder a digital data word selecting a particular tone to decode, and writes to the digital tone encoder a particular data word selecting a tone to be generated. Since the digital tone decoder and digital tone encoder are dedicated to decoding and encoding tones, respectively (and are also external of the microprocessor), they are capable of operating simultaneously such that tones can be simultaneously encoded and decoded.
As mentioned above in connection with GE's prior DELTA and RANGR II MASTR mobile transceivers, relatively inexpensive microprocessors are now fast enough (and otherwise capable) of performing tone encoding and/or tone decoding internally under software control. Digital filtering techniques are commonly used to implement multi-pole filters capable of distinguishing between different tone frequencies. Similarly, it is known how to program a microprocessor to synthesize tones at selected frequencies. Given the spare processing capacity available in RF transceiver control microprocessors --and given the additional flexibility provided when tone encoding and/or decoding functions are performed under software control--it has become desirable in modern RF transceivers to perform all tone encoding and decoding functions with the transceiver microprocessor under software control.
General Electric's DELTA and RANGR II lines of RF transceivers mentioned above are capable of generating CTCSS tones entirely under software control using the transceiver microprocessor, and are also capable of decoding CTCSS tones with the microprocessor under software control. Such transceivers use now-conventional software algorithms (e.g., three-dimensional vector type decode analysis) for reliably, accurately detecting and decoding CTCSS tones of specific frequencies and for encoding (generating) CTCSS tones for transmission. As mentioned, such transceivers operate in only a half-duplex mode and are otherwise not capable of encoding and decoding CTCSS tones simultaneously.
In fact, the basic limitation that a microprocessor is incapable of truly concurrent instruction processing (i.e., it can only process one instruction at a time) leads to a significant problem when attempting to use a single processor for simultaneous CTCSS tone encoding and decoding within a full duplex RF transceiver. A single conventional microprocessor is simply not capable of simultaneously (a) executing an instruction causing a CTCSS tone to be decoded, and (b) executing another instruction causing a CTCSS tone to be encoded. Because received and transmitted CTCSS tones may vary in phase and other timing parameters relative to one another, it may sometimes be that an event relating to CTCSS tone encoding occurs at nearly the same instant in time as an event relating to CTCSS tone decoding in a full duplex environment. A single microprocessor is (because of its inherent incapability of processing instructions in a truly concurrent or parallel manner) incapable of handling such nearly simultaneous events.
For these and other reasons, full software-controlled tone encoding and tone decoding in a single processor architecture without significant tone generation and/or detection circuitry external of the processor has in the past been successful only in contexts where simultaneous tone reception and tone generation is not required. In other contexts (such as full duplex RF transmission systems employing CTCSS signalling) in which simultaneous tone encoding and decoding is required, hardware dedicated to tone encoding and/or decoding has been required in the past--with a microprocessor often supervising the hardware via generating digital control signals but not itself both generating and decoding tones while providing such external hardware components permits simultaneous tone encoding and tone decoding, it also increases the cost and complexity of the RF transceiver. It would be highly desirable to provide a full duplex RF transceiver digital CTCSS tone encoding and decoding arrangement capable of simultaneously encoding and decoding CTCSS tones under software control using a single processor in conjunction with minimal and inexpensive external hardware components.
The present invention provides such a full duplex RF transceiver arrangement including microprocessor that executes CTCSS tone encoding software as well as CTCSS tone decoding software. Simultaneous CTCSS tone encoding and decoding is provided--even though only a single processor is used to perform both functions. In the preferred embodiment, an external memory element (e.g., a flip flop) is used by the processor to help detect transitions of received CTCSS tone signals to be decoded. This memory element (and an associated timing circuit) augments the processor functionality such so that simultaneous software-controlled CTCSS tone encode and decode can be cost-effectively provided in a full duplex RF transceiver using only minimal CTCSS-related encode and decode hardware components external to the processor.